1. Field of the Invention
The present invention generally relates to microwave amplifiers, and more particularly to a low noise, multistage transistor amplifier constructed using microwave integrated circuit techniques.
2. Description of the Prior Art
In the past, the use of tunnel diodes in low noise microwave amplifiers resulted in certain restrictions being placed upon the system designer, the development engineer, and the microwave system itself. These restrictions resulted from the limitations of the device, namely, very low output power before the onset of serious intermodulation distortion, limited gain bandwidth product, the requirement of at least two circular junctions for each 10-12 dB of gain desired in multistage amplifiers, extreme regulation requirements of the supply voltage, moderately high noise figures, and poor reliability and stability.
In spite of the numerous drawbacks of tunnel diode amplifiers, they are often superior to three-terminal or two-port amplifiers such as transistor amplifiers in microwave applications. The limiting factor in transistor amplifiers, besides the transistors themselves, is the close impedance matching required for low noise, broadband operation in the microwave region, for example in the 3.7 to 4.2 GHz satellite communications band. Such amplifiers have typically been constrained to the use of alumina as a substrate and have required multi-element matching networks of short-circuit lines, open-circuit lines, quarter-wavelength transformers, and the like. These additional elements tend to narrow the ultimate bandwidth of the amplifier, and broadband operation can be achieved only at the expense of even more complex matching networks.